1. Technical Field
This application relates generally to the field of steam turbines and a method of increasing service interval periods in a steam turbine.
2. Background Information
Large steam turbines are currently used in electric power plants all around the world for generation of electric power. Such steam turbines often include a high pressure turbine section, an intermediate pressure turbine section, and a low pressure turbine section. In the high pressure turbine section, the steam passing through can often be at very high temperatures, as well as be under very high pressures. Consequently, the components of the high pressure turbine section, and the components of the intermediate and low pressure turbine sections, are subjected to stresses.
One such component is a nozzle block assembly. The nozzle block assembly in a steam turbine directs the flow of steam before the steam contacts the first row of rotating blades. The nozzle block assembly is positioned downstream of the steam inlet nozzle chamber and upstream of the first row of rotating blades. The steam passing through the nozzle block assembly in the high pressure turbine section, for example, has a very high temperature and is under a very high pressure, which temperature and pressure are higher or about the same as that of steam in portions of the steam turbine system located downstream of the high pressure turbine section. The nozzle block assembly in the high pressure turbine section is often held together and attached onto a stationary portion of the steam turbine system by bolts.
Unfortunately, large steam turbines and the components thereof can be contaminated by contaminants, such as alkali metal hydroxides that are present in the water used to produce the steam. Such alkali metal hydroxides may include sodium hydroxide (caustic soda), potassium hydroxide (caustic potash), lithium hydroxide, rubidium hydroxide, and cesium hydroxide. It should be noted that, for purposes of simplicity, sodium hydroxide is primarily discussed in this application as a contaminant. However, it should be understood that any or all of the other alkali metal hydroxides listed, or any other strongly alkaline hygroscopic contaminant, should be considered as possible contaminants, either taken individually or in combination with or excluding sodium hydroxide. Since, over time, steam escapes from the steam turbine system during operation, water must be added to the steam turbine for the generation of more steam. Such contaminants are sometimes introduced into the steam turbine at this time because the replacement water may contain the contaminants. In many modern, high pressure steam turbines, the sodium hydroxide is in a highly concentrated solution at the inlet temperatures of the turbines and can be rather easily spread by the steam throughout the components of the steam turbine system. In some instances, the sodium hydroxide concentration on the parts of the steam turbine may be 90% or greater. Solutions of 90% sodium hydroxide can result in stress corrosion cracking, particularly of high strength materials under stress. The nozzle block bolts, because of the high stresses to which the nozzle block assembly is subjected, are usually made of such high strength materials that are generally susceptible to stress corrosion cracking. Once the corrosion occurs, it is possible to replace the nozzle block bolts. However, unless the steam turbine system has been cleaned of the contamination (a process which can take months or even years), the replacement nozzle block bolts will still be subject to the same sodium hydroxide contamination in the steam turbine system that compromised the original bolts. The replacement parts can then corrode and crack relatively quickly after installation, such as within two or fewer years.